Single-Step Synthesis of Silver Sulfide Nanocrystals in Arsenic Trisulfide

Single-step synthesis of silver sulfide nanocrystals in arsenic trisulfide

Juliana M. P. Almeida,1,2* Chao Lu,1 Cleber R. Mendonça2 and Craig B. Arnold1 1Dep. of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA 2São Carlos Institute of Physics, University of São Paulo, PO Box 369, 13560-970, São Carlos, SP, Brazil * [email protected]

Abstract: Silver sulfide nanocrystals and chalcogenide glasses (ChGs) are two distinct classes of semiconductor materials that have been exploited for new infrared technologies. Each one exhibits particular optoelectronic phenomena, which could be encompassed in a hybrid material. However, the integration of uniformly distributed crystalline phases within an amorphous matrix is not always an easy task. In this paper, we report a single step method to produce Ag2S nanocrystals (NCs) in arsenic trisulfide (As2S3) solution. The preparation is carried out at room temperature, using As2S3, AgCl and propylamine resulting in highly crystalline Ag2S nanoparticles in solution. These solutions are spin-coated on glass and silicon substrates to produce As2S3/Ag2S metamaterials for optoelectronics. ©2015 Optical Society of America OCIS codes: (160.2750) Glass and other amorphous materials; (160.3918) Metamaterials; (310.0310) Thin films. References and links 1. A. L. Rogach, A. Eychmüller, S. G. Hickey, and S. V. Kershaw, “Infrared-emitting colloidal nanocrystals: Synthesis, assembly, spectroscopy, and applications,” Small 3(4), 536–557 (2007). 2. X. Michalet, F. Pinaud, T. D. Lacoste, M. Dahan, M. P. Bruchez, A. P. Alivisatos, and S. Weiss, “Properties of fluorescent semiconductor nanocrystals and their application to biological labeling,” Single Molecules 2(4), 261– 276 (2001). 3. M. Kanehara, H. Koike, T. Yoshinaga, and T. Teranishi, “Indium tin oxide nanoparticles with compositionally tunable surface plasmon resonance frequencies in the near-ir region,” J. Am. Chem. Soc. 131(49), 17736–17737 (2009). 4. J. M. Luther, P. K. Jain, T. Ewers, and A. P. Alivisatos, “Localized surface plasmon resonances arising from free carriers in doped quantum dots,” Nat. Mater. 10(5), 361–366 (2011). 5. S. T. Hussain, S. Abu Bakar, B. Saima, and B. Muhammad, “Low temperature deposition of silver sulfide thin films by AACVD for gas sensor application,” Appl. Surf. Sci. 258(24), 9610–9616 (2012). 6. H. Wang and L. Qi, “Controlled synthesis of Ag2S, Ag2Se, and Ag nanofibers using a general sacrificial template and their application in electronic device fabrication,” Adv. Funct. Mater. 18(8), 1249–1256 (2008). 7. A. I. Kryukov, A. L. Stroyuk, N. N. Zin’chuk, A. V. Korzhak, and S. Y. Kuchmii, “Optical and catalytic properties of Ag2S nanoparticles,” J. Mol. Catal. Chem. 221(1-2), 209–221 (2004). 8. Y. Zhang, G. Hong, Y. Zhang, G. Chen, F. Li, H. Dai, and Q. Wang, “Ag2S quantum dot: a bright and biocompatible fluorescent nanoprobe in the second near-infrared window,” ACS Nano 6(5), 3695–3702 (2012). 9. G. Hong, J. T. Robinson, Y. Zhang, S. Diao, A. L. Antaris, Q. Wang, and H. Dai, “In Vivo Fluorescence Imaging with Ag2S Quantum Dots in the Second Near-Infrared Region,” Angew. Chem. Int. Ed. Engl. 51(39), 9818– 9821 (2012). 10. A. Tubtimtae, K.-L. Wu, H.-Y. Tung, M.-W. Lee, and G. J. Wang, “Ag2S quantum dot-sensitized solar cells,” Electrochem. Commun. 12(9), 1158–1160 (2010). 11. X. Hou, X. Zhang, W. Yang, Y. Liu, and X. Zhai, “Synthesis of SERS active Ag2S nanocrystals using oleylamine as solvent, reducing agent and stabilizer,” Mater. Res. Bull. 47(9), 2579–2583 (2012). 12. K. Terabe, T. Nakayama, T. Hasegawa, and M. Aono, “Formation and disappearance of a nanoscale silver cluster realized by solid electrochemical reaction,” J. Appl. Phys. 91(12), 10110–10114 (2002). 13. K. Terabe, T. Hasegawa, T. Nakayama, and M. Aono, “Quantized conductance atomic switch,” Nature 433(7021), 47–50 (2005). 14. J.-L. Sun, J.-L. Zhu, X. Zhao, and Y. Bao, “Fabrication and photoconductivity of macroscopically long coaxial structured Ag/Ag2S nanowires with different core-to-shell thickness ratios,” Nanotechnology 22(3), 035202 (2011).

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Vlcek, “Structure of pulsed-laser deposited arsenic-rich As-S amorphous thin films, and effect of light and temperature,” J. Non-Cryst. Solids 351(43-45), 3497–3502 (2005). 36. M. V. Kovalenko, R. D. Schaller, D. Jarzab, M. A. Loi, and D. V. Talapin, “Inorganically Functionalized PbS- CdS Colloidal Nanocrystals: Integration into Amorphous Chalcogenide Glass and Luminescent Properties,” J. Am. Chem. Soc. 134(5), 2457–2460 (2012). 37. S. Novak, L. Scarpantonio, J. Novak, M. D. Pre, A. Martucci, J. D. Musgraves, N. D. McClenaghan, and K. Richardson, “Incorporation of luminescent CdSe/ZnS core-shell quantum dots and PbS quantum dots into solution-derived chalcogenide glass films,” Opt. Mater. Express 3(6), 729–738 (2013). 1. Introduction Semiconductor nanocrystals (NCs) and quantum dots are of great interest for their use in a wide range of applications from optoelectronics to biological systems [1, 2]. In contrast to metallic nanoparticles that have a plasmon band in the visible or UV portion of the spectrum, semiconductor nanocrystals exhibit localized surface plasmon resonances in the infrared region [3, 4] making them promising for infrared metamaterials. Silver sulfide is a direct bandgap semiconductor (Eg ~1 eV), commonly used as a solid-state electrolyte, presenting both ionic and electronic conduction [5, 6]. On account of the quantum confinement effect

#241289 Received 22 May 2015; revised 3 Jul 2015; accepted 13 Jul 2015; published 21 Jul 2015 © 2015 OSA 1 Aug 2015 | Vol. 5, No. 8 | DOI:10.1364/OME.5.001815 | OPTICAL MATERIALS EXPRESS 1816 when synthetized at the nanometer scale, indirect transitions have been observed in the range of 0.9 – 1.8 eV and direct transitions are blue shifted, to the range 2.7 – 4.0 eV [7]. Based on these transitions, new applications have been proposed for silver sulfide, such as, NIR emitters for in vivo imaging [8, 9], sensitizers for solar cells [10], and substrates for surface- enhanced Raman scattering (SERS) [11]. In addition, the reversible formation of metallic silver over Ag2S NCs when exposed to an electric field has enabled the development of atomic switching [12, 13], or optical switching materials [14]. Another promising family of semiconductor materials, with interesting optical properties at the infrared region, is chalcogenide glass (ChG). ChGs have high refractive index (n ≈2-3) and high transmittance over to ~11 μm for sulfides, ~15 μm for selenides and beyond ~20 μm for tellurides [15, 16]. Moreover, they present a variety of photosensitive phenomena, including photocrystallization, photodarkening, and photodiffusion, which have motivated numerous researches for decades [16, 17]. Since the first observation of metal photodoping in ChGs, many studies have been performed on the diffusion mechanism of silver in amorphous arsenic trisulfide (As2S3) [18–20]. Basically, by shining light on As2S3, in which a thin metallic layer of silver is deposited, Ag ions can readily dissolve, resulting in a homogeneous doped layer. The mechanism has been explained through the initial formation of Ag−S bond at the silver and ChG interface, followed by the generation of electron-hole pairs and by the mobility of holes toward the silver layer, while Ag+ move in the opposite direction [19]. Recently, we showed the formation of metallic silver nanoparticles in chalcogenide solution using laser ablation of a silver target [21]. Although studies on Ag photodoping in As2S3 have achieved considerable advances, the synthesis of silver sulfide nanocrystals in such material has not been demonstrated yet. In this paper, we report a one-step in situ synthesis of uniformly dispersed Ag2S nanocrystals in As2S3. The raw materials (As2S3 and AgCl) are diluted in an amine solvent and solid-state As2S3:NCs films are prepared by spin-coating the solution on glass or silicon substrates. Such approach enables fabricating samples with arbitrary shapes using soft lithographic processes [22], which is an advantage over other conventional methods like vacuum coating or pulse laser deposition. 2. Materials and methods Solution-processing of ChGs in amine solvents has been long established, and the dissolution mechanism involves an electrophilic substitution reaction, where As atoms are attacked by the lone pair electron associated with the amine group [22–24]. The chemical synthesis employed in this study consists of the dissolution of arsenic trisulfide (alfa aesar 99.999%) in n- propylamine (C3H9N Sigma-Aldric >99%), with a concentration of 133g/L. The dissolution was performed at room temperature, and usually takes 24h to be completed for a solute- solvent ratio of 1 g/7.5 ml. In order to produce Ag2S NCs in- situ, silver chloride (Alfa Aesar 99.997%) was dissolved in n-propylamine (80g/L), and then, both solutions, arsenic sulfide and silver chloride, were mixed together in a ratio of 1ml of As2S3 to 0.25ml of AgCl. The reaction readily occurs, resulting in the formation of silver sulfide nanocrystals in suspension. Due to the absence of stabilizing agents, the reaction also produces an amorphous precipitate. The absorption spectra of the solutions were recorded with a Cary-5000 spectrometer and the nanocrystals were investigated with a Philips CM200 transmission electron microscope (TEM), operating at 200kV, also employed for electron diffraction measurements. Sample preparation for TEM analyses consisted of drop coating a diluted solution (60x with propylamine) over copper grids with a carbon film support. Size distribution was investigated by dynamic light scattering (DLS) measurements using the upper portion of As2S3/AgCl solution. The reaction residue was investigated with a Bruker-D8 x-ray diffractometer, from 30 to 60 ° (2θ), with steps of 0.02 °, using Cu Kα1 radiation. In order to avoid contamination with oxygen, the whole synthesis and solution processing were carried out inside a dry-box with H2O and O2 levels below 1 ppm.

#241289 Received 22 May 2015; revised 3 Jul 2015; accepted 13 Jul 2015; published 21 Jul 2015 © 2015 OSA 1 Aug 2015 | Vol. 5, No. 8 | DOI:10.1364/OME.5.001815 | OPTICAL MATERIALS EXPRESS 1817 Thin films of As2S3 and As2S3:Ag2S NCs were also prepared in a dry-box from their respective solutions by spin-coating. The upper portion of As2S3/AgCl solution was spun at 2000 rpm for 10 - 20s, on glass or silicon substrates. For solvent removal, the thin films were vacuum baked at 60 °C for 1h and then post-baked at 110 °C for 7h. After such annealing, no amine group from the solvent is expected to remain in the film structure [23–25], while pore formation is avoided, once the onset temperature for pore formation has been reported at ~120 °C [26]. Raman spectra were acquired with a LabRAM – Horiba equipment, using a 50 × objective lens, 20s of integration time and excitation at 532 nm from Ar laser. Film thickness is estimated by ellipsometry measurements (M-2000 Woollam) to be approximately 500 nm. 3. Results and discussion

Figure 1 shows the absorption spectra of As2S3 and AgCl dissolved in propylamine individually, and the mixture of both solutions, named As2S3:AgCl. As2S3 solution has a sharp absorption edge at 510 nm, resulting in the typical yellowish color of As2S3 compounds, while silver chloride solution is transparent throughout the entire visible spectrum. Absorption bands at 915, 1044 and 1200 nm are due to the organic solvent. The resulting solution from the mixture (As2S3:AgCl) presents a wide absorption band covering the region 600 - 1000 nm and an absorption edge at 555 nm, conferring a brownish color to the solution. Such features are indicative of the chemical reaction which occurred between the species in solution. The specific wavelength of this absorption suggests the formation of Ag2S in solution as indirect transitions have been reported in this spectral range [7]. However in order to check for the formation of nanocrystals, TEM images are obtained from the diluted solution, as shown in Fig. 2(a) along with electron diffraction measurement. As it can be seen, the chemical reaction produces spherical nanoparticles, uniformly dispersed, with an estimated diameter of 8 nm (obtained using DLS measurements). The diffraction pattern confirms the formation of monoclinic silver sulfide (α-Ag2S), in agreement to ICDD card #00-014-0072, also represented in Fig. 2(a). A representative high-resolution image (HRTEM) is depicted in Fig. 2(b), in which the interplanar distances corresponding to (120), (103) and (031) planes of

Ag2S NPs are seen.

Fig. 1. Absorption spectra of As2S3 and AgCl dissolved in propylamine, and the resulting solution after mixing As2S3/AgCl in a ratio of 1/0.25 ml. The inset shows the variation of the absorption edge (Δλcutoff) over the time of As2S3:AgCl solution.

Fig. 2. (a) TEM image of the NCs disperse in As2S3:AgCl solution and its electron diffraction pattern in which seven crystallographic planes corresponding to monoclinic Ag2S were identified. (b) HRTEM of a single particle, with diameter of 12 nm, where the interplanar

distances match to (120), (103) and (031) planes of Ag2S.

Besides the formation of Ag2S NCs in suspension, a dark precipitate was also observed in the bottom of the reaction vial. XRD and EDS measurements of this precipitate reveled an amorphous phase containing Ag (~4 at.%), As (~38 at.%) and S (~58 at.%). This result suggests that the precipitate is predominantly amorphous As2S3, because the As:S ratio (0.66) is equivalent to the stoichiometric compound. In order to avoid As2S3 precipitation and investigate the nature of the silver portion in the precipitate, the chemical synthesis was performed using a hundred-fold diluted solution of As2S3. The XRD pattern of the resulting precipitate is displayed in Fig. 3, in which unreacted precursor AgCl and monoclinic Ag2S were identified. This confirms the formation and precipitation of silver sulfide crystals. The stability of As2S3:AgCl solution was evaluated over time by its absorption spectrum. The variation of the absorption edge (Δλcutoff) is displayed in the inset of Fig. 1, where negative values indicate changes towards smaller wavelengths over the time. A blue shift of 45 nm in the absorption edge was observed during the first 3h after preparation. For longer periods no significant change was detected, and the solution kept stable for at least 20 days. The blue shift is related to the precipitation process, in which large particles precipitate leaving smaller particles in suspension and a corresponding increase in the apparent bandgap energy due to the quantum size effect [7]. A rough estimative, based on the mass of silver in the precipitated (~2.2 mg), the average diameter of the NP (8 nm) and the density of Ag2S monoclinic crystals (7.2 g/cm3), suggest that the amount of nanoparticles in suspension is around 1016 particles/ml. The formation of Ag2S NCs can be explained based on the sulfidation of silver ions in solution. It is known that the dissolution process of As2S3 results in arsenic sulfide clusters terminated by excess sulfide dangling bonds [22]. Thus, sulfur anions spontaneously react with silver ions that originated from AgCl dissociation, producing nanocrystals of silver + 2- sulfide through the reaction 2Ag + S → Ag2S (ΔH = −2199.5 kJ/mol) [27]. The sulfidation 0 of Ag nanoparticles using H2S exposure is a known method to obtain Ag2S NCs in several systems [28, 29]. However, the presence of sulfur atoms in the chalcogenide solution enables the formation of Ag2S NCs without any gas exposure, enabling a single-step synthesis. It is important to note that no additional source of energy (as temperature or irradiation) is necessary to promote the chemical reaction, configuring a simple and fast way to prepare in situ Ag2S NCs. In addition, this approach can be exploited for the production of other semiconductor sulfide NCs in ChGs to create novel materials for Mid-IR photonics [30, 31].

Fig. 3. XRD pattern of the precipitate formed by mixing the solutions of As2S3 hundredfold diluted and AgCl (regular concentration) in propylamine. Monoclinic Ag2S and cubic AgCl were identified using ICDD. To investigate the structure and physical-chemistry properties of solid-state samples, thin films are prepared from As2S3:AgCl solution (containing Ag2S NCs), and also from the As2S3 solution, for comparison purposes. EDS measurements showed that the films are composed of 63 at.% of S and 37 at.% of As. Thus the As:S ratio is 0.59, indicating an arsenic deficiency when compared to initial As2S3 compound (0.67). Such deficiency has been reported for spin- coated chalcogenide glass, and it is related to the As2S3 dissolution, which leads to the formation of As2Sx clusters terminated with excess of negatively charged S ions [23, 32]. This feature is preserved in the solid phase, resulting in thin films with excess of sulfur atoms. The composition of As2S3/Ag2S NCs films is 3.2 at.% of Ag, 60.7 at.% of S and 36.1 at.% of As. Considering all Ag atoms form Ag2S NCs, the doping amount is half of silver content (1.6 at.% of Ag2S NCs) and the remaining S atoms (59.1 at.%) along with As provide a matrix with As:S ratio of 0.61. Raman spectra of As2S3 and As2S3:NCs films are displayed in Fig. 4. The broad bands indicate the amorphous structure of the films, and are mainly associated with As2S3 and As4S4 structural units, according to the vibrational energy presented in Table 1 [33–35]. As shown in Fig. 4, the presence Ag2S NCs causes minor alterations to the As2S3 structure, indicated by a decreasing shoulder at 297 cm−1 and the vanishing band at 414 cm−1, while peaks at 225 and 330 cm−1 get stronger. Thus, based on the assignments presented in Table 1, we believe that the addition of Ag2S NPs causes a transformation of As2S3 into As4S4 basic units, in agreement with the increase in As content in As2S3:NCs films, seen in the EDS data. The As:S ratio is 0.59 for the undoped film, increasing to 0.61 for the films containing Ag2S NCs. In fact, Iovu et al. described the dissociation 2As2S3→As4S4 + S2 due to rare earth and Mn doping of arsenic sulfide [33]. The preparation of arsenic sulfide films containing nanocrystals of silver sulfide reported herein presents a promising metamaterial for infrared technologies, in which photoactive phenomena associated with semiconductor nanocrystals can be exploited to improve the overall material performance [36, 37].

Fig. 4. Raman shift of As2S3 and As2S3:NCs thin films, in which the amorphous structure was lightly affected by the presence of Ag2S NCs.

Table 1. Raman signature of As2S3 and As2S3:NPs thin films. Peak position (cm−1) Raman signature Ref. 180 As4S4 units [33, 34] 225 As4S4 units, As clusters [34] 297 Asymmetric stretching modes of AsS2/3 pyramids (As2S3 units) [33, 34] 330 Symmetric stretching vibrational mode of AsS2/3 pyramids (As2S3 units) [33, 34] 356 As4S4 units [34] 414 As4S5 [35]

480 S−S stretching vibration in S8 rings [33] 4. Conclusion We have used a wet chemistry approach to produce silver sulfide nanoparticles in chalcogenide solution. The chemical synthesis consists of independently dissolving As2S3 and AgCl in propylamine, and mixing both solutions using the ratio As2S3/AgCl = 1:0.25ml. Such a method results in the spontaneous formation of Ag2S nanocrystals, where the sulfur ions are provided by the As2S3 in solution. The monoclinic structure of Ag2S NCs is confirmed through TEM and XRD analyses. By spin-coating the resulting solution, we are able to produce ~500 nm thick arsenic sulfide films, doped with 1.6 (at.%) Ag2S. The glass network of these films differs from that of an undoped film due to a decrease of As2S3 units in favor of As clusters and As4S4 units. Acknowledgments We acknowledge FAPESP (2013/05350-0) for supporting JMP Almeida’s internship at Princeton University, as well as the PU-USP partnership program. We also thank NSF (EEC- 0540832 and DMR-1420541), FAPESP (2011/12399-0) and CAPES for the financial support. The authors thank the assistance of G. Poirier, Y. Yeh and J. Schreiber for TEM measurements.